The present invention relates to a process fluid recycle system for a compressor assembly having at least a compressor driven from a gear case to recycle process fluid flowing through a shaft seal, from the compressor to the gear case. More particularly, the present invention relates to such a process fluid recycle system in which an anti-back flow compressor is employed to prevent gear oil contained in the gear case from entering the compressor through the shaft seal.
Prior art compressor assemblies employ one or more stages of compression, formed for instance, by a centrifugal compressor driven from an adjacent gear case by a shaft extending through a shaft seal between the stage of compression and the gear case. The shaft seal can be a labyrinth seal that is designed to allow rotation of the shaft while at least inhibiting loss of a process fluid being compressed by the compressor.
The shaft seal itself can be designed to accept a certain flow of the process fluid and therefore, a loss from the compressor. This is done to have a non-contact zero wear gas seal. As a result the gear oil within the gear case will not back flow through the shaft seal, in a direction from the gear case to the stage of compression, and thereby contaminate the process fluid.
As may be appreciated, the flow of process fluid into the gear case must either be vented or recycled back to the stage or stages of compression. When a compressor assembly is used in a refrigeration system, there is no option but to recycle the process fluid in that certain refrigerants are either toxic or potentially destructive to the environment. The potential loss of refrigerant can also degrade the performance of the refrigeration system. For instance, the composition of refrigerants used in mixed gas refrigerant systems, will change due to loss through shaft seals and the like. A typical mixed gas refrigerant is made up of nitrogen, argon, carbon tetrafluoride, pentabromoethane and perfluoropropl methyl ether and such constituents will be lost in unequal amounts due to their different properties. Furthermore, such refrigerants are expensive and any loss of refrigerant is a significant cost penalty to the process.
The prior art provides many examples of compressor assemblies that recycle process fluid flow through the shaft seal back to a low pressure inlet side of the compressor. An example of this can be found in U.S. Pat. No. 6,018,962. In this patent, the compressor assembly is housed in an air tight enclosure. Refrigerant flowing into the gear case mixes with gear oil and a mixture of refrigerant and gear oil collects in the oil sump of the gear case. The mixture is drawn through a demister element to separate the gear oil from the mixture under suction provided by the low pressure side of the compressor. The suction further draws the refrigerant from the demister element back into the low pressure side of the compressor.
In U.S. Pat. No. 4,213,307 the gear case of the compressor assembly is vented to a coalescing filter. The coalescing filter separates the refrigerant from the gear oil. The gear oil, after separation from the refrigerant, is pumped back into a oil sump of the gear case by a jet pump. The refrigerant is drawn from the coalescing filter back to the low pressure side of the compressor. A separate oil pump is used to pump oil to bearings contained within the gear case and also to supply pressurized oil as a motive fluid to the jet pump.
A problem inherent in all of the prior art devices is that when low pressure transients are encountered or during time periods in which the compressor assembly is started or shut down, the pressure within the gear case can be higher than that of the compressor being driven from the gear case. Normally, during operation, the pressure within the compressor is higher than the pressure within the gear case. When such pressure reversal occurs, the gear oil can back flow, that is, be driven through the shaft seal, from the gear case to the compressor to contaminate the refrigerant or other process fluid being compressed.
Furthermore, the illustrative, prior art compressor assemblies, described above, are integrated systems that are not very applicable to large scale installations of compressors or assemblies in which the separate components of the compressor assembly, namely, the motor, gear case, and compressor, are provided with separate enclosures and the components are separately installed on site.
As will be discussed, the present invention provides a system for recycling process fluid flow through a shaft seal of a compressor assembly that is specifically designed to prevent back flow of the gear oil into a compressor. Moreover, any application of the system of the present invention inherently requires very little modification of the components making up the compressor assembly.
The present invention provides a process fluid recycle system for a compressor assembly. The compressor assembly has at least a compressor driven from a gear case. The system acts to recycle process fluid vapor flowing through a shaft seal, from the compressor to the gear case.
In accordance with the present invention, the process fluid recycle system includes at least one coalescing filter to separate oil mist made up of the gear oil from the process fluid vapor. A recycle conduit is connected, at one end, to a low pressure inlet of the compressor assembly. The other end of the recycle conduit is in flow communication with the at least one coalescing filter to return the process fluid vapor to the compressor assembly. Two alternate flow paths are provided to conduct the oil mist and the process fluid vapor from the compressor assembly to the at least one coalescing filter. One of the two alternate flow paths is formed by an anti-back flow compressor in flow communication with the gear case such that operation of the anti-back flow compressor reduces pressure within the gear case below the compressor. The other of the two alternate flow paths is formed by a conduit also in flow communication with the gear case. A valve is located in the conduit to prevent the flow of the oil mist and the process fluid to the gear case during operation of the anti-back flow compressor. A controller activates the anti-back flow compressor to ensure pressure within the gear case is less than that of the compressor, thereby to prevent gear oil from being driven from the gear case into the compressor through the shaft seal.
The compressor assembly can also be provided with an oil sump connected to the gear case such that the oil mist and process fluid vapor collects in a headspace region thereof. The two alternate flow paths are connected to the oil sump so as to receive the oil mist and the process fluid vapor from the headspace region thereof. An oil return pump is connected between the at least one coalescing filter and the oil sump to return the gear oil to the oil sump.
As can be appreciated from the above description, the use of the anti-back flow compressor prevents the back flow of gear oil through the shaft seal that might otherwise occur during start-up and shutdown and other low pressure transients. Moreover, since the recycle system of the present invention is applied to existing compressor assemblies the compressor assemblies do not have to be modified to take advantage of the present invention. In this regard, one or more coalescing filters can be applied to prevent any oil from being recycled back to the compressor because, unlike some prior art designs, the filter does not have to be incorporated into the compressor assembly itself.
The present invention is applicable to multistage compression assemblies and in one aspect recycles process fluids through replication of the process fluid recycle system, described above, for each compressor thereof. In such an assembly, a second compressor is connected in series with a first compressor such that process fluid is initially compressed in the first compressor and is further compressed in the second compressor. The compressor assembly has first and second low pressure inlets to the first and second recycle compressors and first and second gear cases associated therewith.
A recycle system in accordance with this aspect of the present invention has a recycle conduit that is a first recycle conduit connected to the low pressure inlet of the first compressor. A recycle conduit, constituting a second recycle conduit, is connected to a low pressure inlet of the second compressor. The at least one coalescing filter is at least one first coalescing filter in flow communication with the other end of the first recycle conduit. At least one second coalescing filter is in flow communication with the other end of the second recycle conduit. A first of two alternate flow paths in flow communication with the first gear case to conduct the oil mist and the process fluid vapor to the at least one first coalescing filter. A second of the two alternate flow paths is in flow communication with the second gear case to conduct the oil mist and the process fluid vapor to the at least one second coalescing filter. The controller activates each anti-back flow compressor of the first and second of the two alternative flow paths to ensure pressure within the first and second gear case is less than that of the first and second compressor, respectively.
The compressor assembly of a multi-stage unit can also have first and second oil sumps connected to the first and second gear cases such that the oil mist and process fluid vapor collects in first and second headspace regions thereof. The first and second of the two alternate flow paths are connected to the first and second oil sumps so as to receive the oil mist and the process fluid vapor from the first and second headspace regions, respectively. A first oil return pump is connected between the at least one first coalescing filter and the first oil sump to return the gear oil to the first oil sump. A second oil return pump is connected between the at least one second coalescing filter and the first oil sump to return the gear oil to the first oil sump.
The present invention in another aspect is applied to multi-stage compressor assemblies in a more simplified fashion by combining elements. For instance in a multi-stage compressor assembly, the process fluid enters the first compressor through the low pressure inlet thereof, which thus constitutes a system inlet for the compressor assembly. The one end of the recycle conduit is connected to the system inlet. A first of two alternate flow paths is in flow communication with the first gear case to conduct the oil mist and the process fluid vapor to the at least one coalescing filter. A second of the two alternate flow paths is in flow communication with the second gear case to also conduct the oil mist and the process fluid vapor to the at least one coalescing filter. The controller activates each anti-back flow compressor of the first and second two alternate flow paths to ensure pressure within the gear case is less than that of the first and second compressors.
The compressor assembly can be provided with first and second oil sumps connected to the first and second gear cases such that the oil mist and process fluid vapor collects in first and second headspace regions thereof. The first and second of the two alternate flow paths are connected to the first and second oil sumps so as to receive the oil mist and the process fluid vapor from the first and second headspace regions, respectively. An oil return pump is connected between the at least one coalescing filter and the first and second oil sump to return the gear oil to the first and second oil sump.
Alternatively, first and second phase separators are connected to the first and second gear cases to separate the oil mist and the process fluid vapor from the gear oil. The compressor assembly also has a common oil sump connected to the first and second phase separators to receive the gear oil therefrom. The first and second of the two alternate flow paths are connected to the first and second phase separators to receive the oil mist and the process fluid vapor therefrom. An oil return pump is connected between the at least one coalescing filter and the common oil sump to return the gear oil to the common oil sump.
In a yet further aspect of the present invention involving its application to multi-stage compressors, still further simplification is possible. In such aspect of the present invention, the one end of the recycle conduit is connected to the system inlet. The compressor assembly is provided with a common oil sump connected to the first and second gear cases such that the process fluid vapor and the oil mist collecting in a headspace region thereof. This allows the two alternate flow paths to be connected to the common oil sump so as to receive the process fluid vapor and the oil mist from the headspace region. An oil return pump is connected between the at least one coalescing filter and the common oil sump to return the gear oil to the first and second oil sumps.
In all aspects of the present invention, an oil vapor adsorption trap and a water vapor adsorption trap can be interposed between the at least one coalescing filter and the conduit or each of the at least one first and second coalescing filters and each of the first and second recycle conduits. Furthermore, the controller can be a pressure differential switch connected to the anti-back flow compressor. In aspects of the invention involving multi-stage compression, the controller can comprise two pressure differential switches each respectively connected to the anti-back flow compressor of the first and second two alternate flow paths. Two pressure differential switches are positioned to react to pressure differentials between the first gear case and the first compressor and the second gear case and the second compressor.